IRISH POTATO (Solanum tuberosum L.)

Potato is a temperature crop which is being increasingly cultivated in the tropics in areas with high elevations and cool temperatures.

It is referred to as Irish potato to differentiate it from the sweet potato though it did not originate from Ireland.

It is regarded as a valuable vegetable and generally highly priced compared with other tuber crops because of its high cost of production and limited ecology in the tropics.

About 75% of the annual production of 500,000 tons in Nigeria comes from jos plateau.

It originated in the highlands of Peru and Bolivia in south America. Spanish explorers introduced it into Europe and it eventually became the staple food of the Irish. It spread to other parts of the world from Europe.

Potato is not a tropical crop. It is essentially a crop of cool moist regions and grows best in climates where cool nights alternate with warm days during the period of tuber formation.

It requires an optimal growing temperature between 15 and 21°C and a well a distributed rainfall over a period of 3 months and two weeks. It is relatively sensitive to frost.

Tuberisation is greatest when the soil temperature is 17°C; above this, tuber formation is totally inhibited.

Potato is responsive to day length. Long days, high temperatures and high nitrogen favor the heavy (vegetative) growth of the plant, while short days, cool temperatures or deficiency of nitrogen favor early tuberisation.

The crop can grown in a wide variety of soils but a well-drained soil is essential for good growth as it offers least resistance to the expanding tubers.

It can tolerate acidic conditions, down to pH 5.0.

In Nigeria, it is grown mainly in Jos plateau. It is also grown in Mambilla, Biu plateaux and areas in the country north of latitude 11° N.

In rainy and dry seasons, jos, Mambilla and Biu Plateaux satisfy the conditions for potato growth.

Most areas north of latitude 11°N satisfy this requirement during the harmattan months (November- February) only.

It is a tuber bearing herbaceous perennial but is treated as an annual under cultivation.

The aerial part of the stem is erect in the early stages of growth but later it becomes more spreading.

The underground portion of the stem is more or less rounded in the soil sending forth horizontal branches (stolons) that arisen from the axillary buds.

Adventitious roots are produced in groups of three or four at the nodes of the main underground stem as well as the stolons.

The tuber is the short greatly enlarged apical portion of the stolon, full of stored food.

Morphologically, it is a shortened thick stem bearing a group of buds protected by scale-like leaves which are eventually shed, leaving rudimentary leaf scars (eyebrows) or ridges.

Each ‘eye’ consists of a cluster of at least three buds lying in a slight depression, representing a lateral branch with under-developed internodes.

The ‘eyes’ arranged spirally around the tuber and are more crowded towards the apical end than towards the basal end of the tuber.

The size, shape and color of the tubers vary greatly, the most common colors being white, red, yellow or purple.

The skin may be smooth or rough and can be easily peeled off. The greater part of the proteins, minerals, tannins, cystals and pigment (in colored varieties) is localised in the outer layers of the cortex.

Deep peeling of potatoes should always be avoided as it removes the valuable nutritional ingredients.

The first few leaves few leaves developing from the ‘seed’ piece are usually simple but subsequent ones are compound.

The leaflets are more or less opposite and are densely hairy when when young but at a maturity the hairs are confined to the midribs and lateral veins.

The leaves ate spirally arranged on the main stem.

Flowers may or may not be produced, depending upon the variety, when present, they are borne in terminal clusters.

These include Diamant and ‘Up to date’. Pest and disease resistant cultivars include: RC 77J, VC 783-2, B 949 -3, RC 676-2, RC – 785-1 and VC 730-1

Cultural practices
Cultivation is by use of ‘seed tubers‘ free from disease which are planted while it cut into sections or setts.

Large ‘seed potatoes’ are cut into several section, each having at least one complete ‘eye’.

They should be cut at right angles to the main axis to eliminate apical dominance.

Potatoes may be planted entirely by hand or by using a potato planter, generally on ridges 0.9m or 0.75m apart and spaced 20-30cm within the rows.

For the wet season crop which is dependent on rainfall, plant early in April/may when the rains become reliable.

The dry season crop is planted in November followed by irrigation.

Fertilizers application will depend on the fertility status of the soil; generally about 200-300 kg/ha NPK should be applied.

Weed control can be done manually at 3 and 7 weeks or the herbicide sencor 70 WP (Metribuzin) can be applied pre-emergence at the rate of 0.7kg a.I/ha.

Cupravit at 5.5kg/ha can also be applied to protect the crop against blight.

Potatoes are harvested at 3 months and some weeks after planting. Hand digging is still practiced but in agriculturally advanced countries, the crop is harvested mechanically.

Yields are generally small in the tropics; between 15-20 t/ha of tubers can be obtained.

Harvested tubers can be stored in a cool environment at about 4°C after washing out the soil.

They can be stored in dry form; dried and dehydrated potatoes retain all the ingredients originally present in the tuber with the exception of water.

Chemical composition
Raw tuber contains 70-80% water, 17-18% starch, about 2% protein and 25mg% vitamin C.

Green potatoes contain a poisonous glucoside, solanin.

1. Fresh tubers are usually consumed after boiling, mashing, baking, frying or roasting. It can also be made into potato crisps or chips.

2. Potato flour (up to 4%) can be mixed with wheat flour for bread baking.

3. Starch can be extracted industrially from potatoes. Potato starch is used chiefly for sizing cloth and paper, in the laundry industry and in various food preparations.

4. It can be used for production if industrial alcohol by fermentation with yeast. For example, vodka, a Russian alcoholic beverage, is prepared by the fermentation of cooked potatoes.

5. The peels of potato tubers are rich in nutrients and can be usedbas livestock feed.

Most serious is potato blight caused by Phytophthora infestans and this can be controlled with fungicides; e.g ridomil MZ applied biweekly at 2.5 kg/ha.

Bacterial wilt is caused by Pseudomonas solanacearum and can be controlled by phytosanitary measures such as crop rotation and strict control of the seed potatoes used.

Agrotis spp. (cutworms) and the potato tuber moth (Phthorimacea operculella) are important pests.